This will start a netcat process listening on port 666. If you are able connect to your your server, netcat will receive the data being sent and spit it out to the screen (it may look like random garbage, so you might want to redirect it to a file).

Telnet will make a tcp connection to a remote ip/port to test connectivity. If it times out, it's not reaching the host (maybe the firewall is blocking it). If connection is refused, it's reaching the host, but either the service is not listening on that port, or it's locked.